Field
Exemplary embodiments relate to wireless communication, and more particularly, to a method and an apparatus for controlling a radio link in a wireless communication system supporting dual connectivity.
Discussion of the Background Art
Cellular is a concept proposed to overcome limitation of a service area and limitation of a frequency and a subscriber capacity. This is a scheme that provides call coverage by changing a single high-output base station to multiple low-output base stations. That is, a mobile communication service area is divided into several small cells to allocate respective different frequencies to adjacent cells and the same frequency band is used in two cells which are significantly far away from each other and do not interfere with each other to spatially reuse the frequency. Alternatively, a scheme may also be used which divides the mobile communication service area into several small cell units, but allocates the same frequency to adjacent cells, however, controls interference among the cells to be removed.
Meanwhile, particular many communication demands are generated in a specific area such as a hotspot in the cell and receiving sensitivity of radio waves may deteriorate in a specific area such as a cell edge or a coverage hole. With the development of wireless communication technology, small cells, for example, a pico cell, a femto cell, a micro cell, a remote radio head (RRH), a relay, a repeater, and the like are together installed in a macro cell for the purpose of enabling communication in regions such as the hotspot, the cell edge, the coverage hole, and the like. The small cells may be positioned inside or outside the macro cell. In this case, the small cell is positioned at a location which the macro cell does not reach, outdoor, or at an office. The network is called a heterogeneous network (HetNet). In this case, heterogeneous networks need not use different radio access schemes. In a heterogeneous network environment, the macro cell is a cell having relatively large coverage and the small cell such as the femto cell and the pico cell is a cell having relatively small coverage. The macro cell and the small cell may distribute the same traffic or take charge of transmitting traffic of different QoSs. In the heterogeneous environment, coverage overlapping occurs among multiple macro cells and small cells.
In the heterogeneous environment, as one of cell planning techniques for distributing an excessive load or a load requiring a specific QoS to the small cell without a handover procedure and efficiently transmitting data, a dual connectivity technique is introduced. In terms of a user equipment, the dual connectivity may be a technique for providing a more efficient scheme in terms of transmission/reception transmission rate. For example, the user equipment may transmit/receive services from two or more serving cells. In this case, the respective serving cells may belong to different base stations. As described above, at the region where the coverage of the macro cell and the coverage of the small cell overlap with each other, the user equipment may be simultaneously connected (signaling connection) to the macro cell and the small cell or simultaneously use (user traffic transmission) the macro cell and the small cell. This may be referred to as the dual connectivity. That is, the user equipment are wirelessly connected with two or more different base stations (for example, a macro base station including the macro cell and a small base station including the small cell) through different frequency bands or the same frequency band based on the dual connectivity technique to transmit/receive the services. Alternatively, the user equipment is wirelessly connected with two or more different base stations through the same frequency band to transmit/receive the services.
Since the user equipment supporting the dual connectivity may simultaneously use the macro cell and the small cell, two radio links can be maintained. In one single connectivity situation in the related art, the user equipment may monitor one radio link and when a problem occurs in the corresponding link, a radio link failure (RLF) may be declared. However, in the dual connectivity to maintain two radio links, even though a problem occurs in one radio link of the user equipment, the other radio link can still be available. When the problem occurs in one radio link, it is not preferable in terms of network performance to unconditionally declare the radio link failure and perform a radio resource control (RRC) reestablishment procedure even though the other radio link is available. Accordingly, a radio link control method considering the dual connectivity is required.